European Tracer Experiment Research Articles

Evaluating the ability of a numerical weather prediction model to forecast tracer concentrations during ETEX 2

In this paper the meteorological processes responsible for transporting tracer during the second ETEX (European Tracer EXperiment) release are determined using the UK Met Office Unified Model (UM). The UM predicted distribution of tracer is also compared with observations from the ETEX campaign. The dominant meteorological process is a warm conveyor belt which transports large amounts of tracer away from the surface up to a height of 4 km over a 36 h period. Convection is also an important process, transporting tracer to heights of up to 8 km. Potential sources of error when using an operational numerical weather prediction model to forecast air quality are also investigated. These potential sources of error include model dynamics, model resolution and model physics. In the UM a semi-Lagrangian monotonic advection scheme is used with cubic polynomial interpolation. This can predict unrealistic negative values of tracer which are subsequently set to zero, and hence results in an overprediction of tracer concentrations. In order to conserve mass in the UM tracer simulations it was necessary to include a flux corrected transport method. Model resolution can also affect the accuracy of predicted tracer distributions. Low resolution simulations (50 km grid length) were unable to resolve a change in wind direction observed during ETEX 2, this led to an error in the transport direction and hence an error in tracer distribution. High resolution simulations (12 km grid length) captured the change in wind direction and hence produced a tracer distribution that compared better with the observations. The representation of convective mixing was found to have a large effect on the vertical transport of tracer. Turning off the convective mixing parameterisation in the UM significantly reduced the vertical transport of tracer. Finally, air quality forecasts were found to be sensitive to the timing of synoptic scale features. Errors in the position of the cold front relative to the tracer release location of only 1 h resulted in changes in the predicted tracer concentrations that were of the same order of magnitude as the absolute tracer concentrations.

On the importance of the meteorological coupling interval in dispersion modeling during ETEX-1

Traditionally, transport and dispersion models are offline coupled to meteorological drivers, receiving pre-processed output at regular coupling intervals. However, today meteorological models have reached urban and cloud resolving scales and online models integrating meteorological and dispersion models have been developed. In this study the online coupled model, Enviro-HIRLAM, which can also run in offline mode, was used to compare online and offline representations of meso-scale disturbances. The online model was evaluated using data from the first European Tracer Experiment (ETEX-1) and produced satisfactory results. Meso-scale influences during the simulation pertube the plume during long-range transport, leading to a double peak structure at a specific measurement station. The meso-scale influence was investigated by varying the offline coupling interval which was shown to be important in constraining the influence of meso-scale disturbances on plume structure in coarse resolution.

Inverse modelling of atmospheric tracers: non-Gaussian methods and second-order sensitivity analysis

Abstract. For a start, recent techniques devoted to the reconstruction of sources of an atmospheric tracer at continental scale are introduced. A first method is based on the principle of maximum entropy on the mean and is briefly reviewed here. A second approach, which has not been applied in this field yet, is based on an exact Bayesian approach, through a maximum a posteriori estimator. The methods share common grounds, and both perform equally well in practice. When specific prior hypotheses on the sources are taken into account such as positivity, or boundedness, both methods lead to purposefully devised cost-functions. These cost-functions are not necessarily quadratic because the underlying assumptions are not Gaussian. As a consequence, several mathematical tools developed in data assimilation on the basis of quadratic cost-functions in order to establish a posteriori analysis, need to be extended to this non-Gaussian framework. Concomitantly, the second-order sensitivity analysis needs to be adapted, as well as the computations of the averaging kernels of the source and the errors obtained in the reconstruction. All of these developments are applied to a real case of tracer dispersion: the European Tracer Experiment [ETEX]. Comparisons are made between a least squares cost function (similar to the so-called 4D-Var) approach and a cost-function which is not based on Gaussian hypotheses. Besides, the information content of the observations which is used in the reconstruction is computed and studied on the application case. A connection with the degrees of freedom for signal is also established. As a by-product of these methodological developments, conclusions are drawn on the information content of the ETEX dataset as seen from the inverse modelling point of view.

High‐resolution reconstruction of a tracer dispersion event: application to ETEX

AbstractIn a previous two‐part paper, new methods for reconstructing the source of an atmospheric tracer at regional scale were developed. Specifically, the ‘maximum entropy on the mean’ (MEM) method was extended to large (though linear) data assimilation problems. Tests using twin experiments and a limited subset of the data from the European Tracer Experiment (ETEX) were performed. Although temporal reconstruction knowing the location of the source was satisfying, a full three‐dimensional reconstruction with real data was still out of reach.In this paper, using the MEM method and some of its refinements, a reconstruction using all ETEX‐I measurements at a resolution of 1.125 × 1.125 × 1 h is shown to be possible. This allows for a reconstruction of the full dispersion event. The MEM retrieval of the tracer plume using what is believed to be a good prior is then compared to retrievals using other priors, including Gaussian priors. Eventually, a reconstruction using all data sequentially in time (rather than all together) is obtained. This helps define what a maximum‐entropy filter applied to sequential data assimilation of a linear tracer should be able to do, with a view to an efficient emergency response in case of an accidental release of pollutant. Copyright © 2007 Royal Meteorological Society

Data assimilation in meteorological pre-processors: Effects on atmospheric dispersion simulations

In previous work [Kovalets, I., Andronopoulos, S., Bartzis, J.G., Gounaris, N., Kushchan, A., 2004. Introduction of data assimilation procedures in the meteorological pre-processor of atmospheric dispersion models used in emergency response systems. Atmospheric Environment 38, 457–467.] the authors have developed data assimilation (DA) procedures and implemented them in the frames of a diagnostic meteorological pre-processor (MPP) to enable simultaneous use of meteorological measurements with numerical weather prediction (NWP) data. The DA techniques were directly validated showing a clear improvement of the MPP output quality in comparison with meteorological measurement data. In the current paper it is demonstrated that the application of DA procedures in the MPP, to combine meteorological measurements with NWP data, has a noticeable positive effect on the performance of an atmospheric dispersion model (ADM) driven by the MPP output. This result is particularly important for emergency response systems used for accidental releases of pollutants, because it provides the possibility to combine meteorological measurements with NWP data in order to achieve more reliable dispersion predictions. This is also an indirect way to validate the DA procedures applied in the MPP. The above goal is achieved by applying the Lagrangian ADM DIPCOT driven by meteorological data calculated by the MPP code both with and without the use of DA procedures to simulate the first European tracer experiment (ETEX I). The performance of the ADM in each case was evaluated by comparing the predicted and the experimental concentrations with the use of statistical indices and concentration plots. The comparison of resulting concentrations using the different sets of meteorological data showed that the activation of DA in the MPP code clearly improves the performance of dispersion calculations in terms of plume shape and dimensions, location of maximum concentrations, statistical indices and time variation of concentration at the detectors locations.

Verification of the Lagrangian particle model using the ETEX experiment

A Lagrangian particle model named LADAS (Long-range Accident Dose Assessment System) has been developed to understand the characteristics of the long-range transport and diffusion of a radioactive material released into the atmosphere. The developed numerical model was compared with the results of European Tracer Experiment (ETEX) which is a long-range field tracer experiment. Several simulations were performed to investigate the effects according to the variation of the mixing height. As a comparative study, the calculated concentration distributions agreed well in the case of the usage of the mixing height calculated by the Richardson number than the usage of the constant mixing height in the computational domain. The calculated particle cloud represented well the phenomenon that was transported north-westerly from a release point such as the ETEX results.

A dispersion modelling system SILAM and its evaluation against ETEX data

This paper presents the SILAM dispersion modelling system that has been developed for solving various forward and inverse dispersion problems. The current operational version is based on a Lagrangian dispersion model that applies an iterative advection algorithm and a Monte Carlo random-walk diffusion representation. The system can utilize meteorological data from either the HIRLAM or ECMWF numerical weather prediction models. We present an evaluation of SILAM against the data of the European Tracer Experiment (ETEX). The model showed an overall time correlation coefficient of 0.6 (over 150 stations), with specific values for the two ETEX measurement arcs of 0.75 and 0.74, respectively. The number of well-reproduced observation sites are 55, 37, and 40—for a Figure of Merit in Time of >0.2, a correlation coefficient of >0.7, and mean observed and modelled values being within a factor of 2, respectively. We have also investigated the sensitivity of the model to the meteorological input data and model setup. The most important factors with regard to the model performance were (i) the selection of the meteorological input data set and (ii) the method used for the atmospheric boundary layer height estimation. The study allowed selection of the optimum setup for the operational model configuration. We also tried to find explanations for the successes and failures of the specific methodologies in order to facilitate broader conclusions on their applicability in emergency dispersion modelling.

Comparisons of transport and dispersion model predictions of the european tracer experiment: area- and population-based user-oriented measures of effectiveness

Abstract In October 1994, a tracer gas was released from a location in northwestern France and tracked at 168 sampling locations in 17 countries across Europe. This release, known as the European Tracer Experiment (ETEX), resulted in the collection of a wealth of data. This paper applies a previously described user-oriented measure of effectiveness (MOE) methodology to evaluate the predictions of 46 models against the long-range ETEX observations. The paper extends previous work by computing MOE values that are based on “true” areas (e.g., in square kilometers) and on actual European population distributions. In this way, assessments of model predictions of ETEX are placed in a possible operational context. The predictive performance of the models were assessed with nominal, area-, dosage-, and population-based MOE values and ranked using a few notional user-based scoring functions. This study finds that the rankings of some models—in particular, several of the model predictions that were ranked in the top 10—are relatively insensitive to the particular MOE technique used. This robust behavior, with respect to analysis assumptions, is regarded as an important feature of model performance.

Geostatistical investigation of ETEX-1: Structural analysis

The surface concentration data collected during the European Tracer Experiment (ETEX) are analysed for the first time by means of geostatistical techniques. The aim of the analysis is to determine the self-consistency of the data, to identify anomalous behaviours of stations with respect to the spatial and time structure of the tracer cloud measurements and to characterise the correlation structure of the measured cloud. The so-called structural analysis is usually regarded as the investigation step to be conducted prior to any interpolation procedure and mapping. Given the relevance of the ETEX dataset, one of the few existing sets of information related to controlled long-range dispersion of tracers, the analysis presented here should be regarded as a way to acquire further insight into the observed data characteristics.

Grid resolution dependence in the reconstruction of an atmospheric tracer source

Abstract. Methods for reconstructing sources of inert atmospheric tracers from ground measurements are currently studied, tested, and even implemented (in accident-type radionuclide release backtracking, for retrieval of carbon fluxes). Often the retrieved source exhibits a very strong and unrealistic (therefore unwanted) influence by the observation sites. This problem is shown not to be an intrinsic flaw of the reconstruction methods but rather due to the specifics of the atmospheric dispersion of a tracer, to the location of the receptors and to the expected source location. It is increasingly pronounced as the grid resolution for the source is improved, and we show how this translates mathematically. We rely on the general framework of inversion methods based on the maximum entropy on the mean principle. Those methods are well suited for accident-type tracer release problems. The dependence of the reconstruction on grid resolution is investigated both analytically and numerically, in conjunction with the issue of receptor influence. Two examples of synthetic experiments are given. The first one is a one-dimensional toy model which quantitatively validates the approach. The second one is based on the European Tracer Experiment and agrees well with the results obtained here. Finally, a generalization of the formalism is proposed so as to study the performance of reconstructions when observation and possibly model errors are present.

Experimental verification for real-time environmental emergency response system: WSPEEDI by European tracer experiment

Abstract Japan Atomic Energy Research Institute has developed an emergency response system WSPEEDI to forecast long-range atmospheric dispersions of radionuclides discharged into the atmosphere. The latest version of WSPEEDI consists of an atmospheric dynamic model MM5 for calculating meteorological fields and a particle random-walk model for atmospheric dispersion. The performance of WSPEEDI was evaluated by data obtained from a field tracer experiment over Europe (ETEX) in this paper. The model validation was done with respect to the following points: (1) the dependence of model accuracy on the temporal and spatial resolutions of the meteorological fields and (2) the superiority of an atmospheric dynamic model over a mass-consistent wind model. Regarding (1), it was shown that the calculation accuracy of the new version with high temporal resolution was improved, especially at the edge of the plume. Moreover, although the increase in horizontal spatial resolution of the old version had no substantial effect on the model performance, increase in horizontal resolution of the new version contributed to the significant improvement of the calculation accuracy. These results showed that the dynamically calculated meteorological field with the spatial resolution of the meso- β – γ scale greatly improved calculation accuracy.

Application of user-oriented measure of effectiveness to transport and dispersion model predictions of the European tracer experiment

In October 1994, the tracer gas perfluoro-methyl-cyclohexane (PMCH) was released over a 12-h period from a location in northwestern France and tracked at 168 sampling locations in 17 countries across Europe. This release, known as the European Tracer Experiment (ETEX), resulted in the collection of a wealth of data. The authors have obtained 46 sets of transport and dispersion predictions associated with models from 17 countries from the Joint Research Centre of the European Commission as well as the PMCH sampling data. This paper compares predictions and observations, paired in space and time, via the application of a previously developed user-oriented measure of effectiveness (MOE). First, model predictions of ETEX are compared using the MOE and “ranked” by a variety of notional user criteria. The development of these criteria are described. Next, the sensitivity of MOE estimates to any single sampler location is analyzed and the use of the MOE to explore the time-dependence of model performance is described. Finally, the model prediction rankings described in this work are briefly compared with previous related research.

Pollutant transport schemes integrated in a numerical weather prediction model: model description and verification results

AbstractRegional air pollution models are usually off‐line coupled with numerical weather prediction models. The present study, however, focuses on on‐line coupled modelling, for which the air pollution model is an integral part of the meteorological model. To this purpose, simulations of the first European Tracer Experiment (ETEX), which supplies the best suited verification data for dispersion modelling of a passive tracer, have been performed using versions of the Danish Meteorological Institute High Resolution Limited‐Area Model (DMI‐HIRLAM). Simulation results have been compared with observed values as well as with results from other models.The operational version of DMI‐HIRLAM using well‐known basic advection schemes has been tested, showing the need for improvements for air pollution purposes. The fourth‐order Bott algorithm for advection has thus been implemented in the semi‐Lagrangian version of DMI‐HIRLAM. Due to its practically perfect mass conservation properties and its low computational cost, this method turns out to be efficient for simulations of transport of pollutants in the atmosphere. This represents a first effort towards a fully integrated air pollution model, e.g. for forecasting purposes. Copyright © 2004 Royal Meteorological Society

Ensemble dispersion forecasting—Part II: application and evaluation

The data collected during the long-range European tracer experiment (ETEX) conducted in 1994, are used to estimate quantitatively the ensemble dispersion concept presented in Part I. The modeling groups taking part to the ENSEMBLE activities (see, Part I) repeated model simulations of the dispersion of ETEX release 1 and the model ensemble is compared with the monitoring data. The scope of the comparison is to estimate to what extent the ensemble analysis is an improvement with respect to the single model results and represents a superior analysis of the process evolution.

Description and performance of Bulgarian Emergency Response System in casе of nuclear accident (BERS)

A PC-oriented Emergency Response System in case of nuclear accident (BERS) is developed and works operationally in the National Institute of Meteorology and Hydrology (NIMH) with the Bulgarian Academy of Sciences. The creation and development of BERS was highly stimulated by the ETEX (European Tracer Experiment) project, followed by the European RTMOD (Real-Time Modelling) project. BERS comprises two main parts – the operational and the accidental one, which are applied both to region "Europe" and region "Northern Hemisphere" (versions E and NH). The operational part runs automatically. It makes search for the necessary meteorological data received in NIMH via the Global Telecommunication System (GTS) of the World Meteorological Organisation (WMO), prepares the input meteorological file used by both trajectory and dispersion models, creates archives with meteorological data base, runs the trajectory models, visualises the results obtained and uploads the maps of trajectories for specified nuclear power plants (NPP) to the web site of NIMH. The operational part runs every 12 hours, after the new meteorological information is received. The accidental part is activated manually when a real radioactive releases in the environment occur or during emergency exercises. Two Bulgarian dispersion models – LED and EMAP are the cores of the accidental part. The concentration and accumulated deposition fields are output by these models. After visualisation the concentration and deposition maps are uploaded to the ftp-server of NIMH for use by the interested institutions. In the paper, the BERS overall structure is described and a number of applications are shown including several runs in the frame of the European 5th Framework Programme Project ENSEMBLE.

Modelling transport and deposition of caesium and iodine from the Chernobyl accident using the DREAM model

Abstract. A tracer model, DREAM (the Danish Rimpuff and Eulerian Accidental release Model), has been developed for modelling transport, dispersion and deposition (wet and dry) of radioactive material from accidental releases, as the Chernobyl accident. The model is a combination of a Lagrangian model, that includes the near source dispersion, and an Eulerian model describing the long-range transport. The performance of the transport model has previously been tested within the European Tracer Experiment, ETEX, which included transport and dispersion of an inert, non-depositing tracer from a controlled release. The focus of this paper is the model performance with respect to the total deposition of 137Cs, 134Cs and 131I from the Chernobyl accident, using different relatively simple and comprehensive parameterizations for dry- and wet deposition. The performance, compared to measurements, of using different combinations of two different wet deposition parameterizations and three different parameterizations of dry deposition has been evaluated, using different statistical tests. The best model performance, compared to measurements, is obtained when parameterizing the total deposition combined of a simple method for dry deposition and a subgrid-scale averaging scheme for wet deposition based on relative humidities. The same major conclusion is obtained for all the three different radioactive isotopes and using two different deposition measurement databases. Large differences are seen in the results obtained by using the two different parameterizations of wet deposition based on precipitation rates and relative humidities, respectively. The parameterization based on subgrid-scale averaging is, in all cases, performing better than the parameterization based on precipitation rates. This indicates that the in-cloud scavenging process is more important than the below cloud scavenging process for the submicron particles and that the precipitation rates are relatively uncertain in the meteorological model compared to the relative humidity. Relatively small differences are, however, seen in the statistical tests between the three different parameterizations of dry deposition.

Evaluation of the Operational Multiscale Environment Model with Grid Adaptivity against the European Tracer Experiment

Abstract The Operational Multiscale Environment Model with Grid Adaptivity (OMEGA) is a multiscale nonhydrostatic atmospheric simulation system based on an adaptive unstructured grid. The basic philosophy behind the OMEGA development has been the creation of an operational tool for real-time aerosol and gas hazard prediction. The model development has been guided by two basic design considerations in order to meet the operational requirements: 1) the application of an unstructured dynamically adaptive mesh numerical technique to atmospheric simulation, and 2) the use of embedded atmospheric dispersion algorithms. An important step in proving the utility and accuracy of OMEGA is the full-scale testing of the model using simulations of real-world atmospheric events and qualitative as well as quantitative comparisons of the model results with observations. The main objective of this paper is to provide a comprehensive evaluation of OMEGA against a major dispersion experiment in operational mode. Therefore, O...

A More Extensive Investigation of the Use of Ensemble Forecasts for Dispersion Model Evaluation

Abstract An ensemble forecast is used as input to a Lagrangian particle dispersion model to study the effect that analysis errors in the numerical weather prediction assimilation cycle have on dispersion modeling. The wind and temperature fields from a control forecast are first compared with radiosonde measurements to quantify the quality of the forecast. It predicts, on average, a less stable atmosphere near the surface, whereas the average deviations in wind speed and wind direction are small. The dispersion model then simulates a tracer puff release from the European Tracer Experiment (ETEX), by using the ensemble forecast as input. The spread in the ensemble forecast is calculated by comparing the predicted puff concentrations for each ensemble member with those from the control forecast. It was found that ensemble members that were close to the control forecast when one statistical measure was calculated were not necessarily close when another was used. The ensemble spread is then quantified by comp...

Numerical modelling of transport, dispersion, and deposition — validation against ETEX-1, ETEX-2 and Chernobyl

A comprehensive, high-resolution, 3-dimensional tracer model, DREAM, has been developed for studying transport, dispersion, and deposition of air pollution caused by a single but very strong source. The model is based on a combination of a Lagrangian short-scale puff model and an Eulerian long-range transport model. The meteorological meso-scale model MM5V1 is used as a driver for the transport model. The tracer model and the numerical implementation of the model is described in this paper. The Eulerian transport model has been splitted into different submodels. The advantages of treating the different physical processes on different scales by using different numerical algorithms will be discussed. The model has been run and validated against measurements from the two ETEX releases (the European Tracer EXperiment) and the Chernobyl accident. 2-D and 3-D visualizations together with some comparisons of model results with measurements will be presented and discussed. The combined model is able to reproduce dosages within a factor of 2–3 in the worst case and arrival times within 2–3 h, compared to measurements from ETEX-1 and Chernobyl. This is within the currently achievable limits of accuracy in long-range dispersion modelling, according to the ETEX-1 experiment.

Simulation of the second ETEX release in the proximity of the source

The complexity of the weather situation during the second release of ETEX (European Tracer EXperiment) resulted in generally poor performances of the long-range dispersion models employed in the real time simulation of the release. On the other hand, much more detailed meteorological information than those available to the real time modellers were assembled in the course of the experiment and stored in a data bank. This has allowed the possibility of investigating with a much finer mesh interval the mesoscale flow over a few hundreds kilometres range from the source, using a model that could take into account the terrain topography. The resulting flow was used as input to a Lagrangian particle and a Lagrangian puff model that simulated the cloud development for the first hours after the release start. The comparison between calculated and measured tracer concentrations was carried on for a number of sites close to the release site. The result indicated that some of the peculiarities in the dispersion found by the experiment were reproduced, thereby enhancing the importance of this type of approach.